I heard "Ne kantu!" Don't sing... But it is "Ni"...
Why not 'We sing!' ?
This would be ni kantas.
Why we should sing is correct but shall is not?
Submitted "we shall sing" as an acceptable translation. Would that be correct?
If you're still here, no, "We shall sing" is not correct here. It's just another way of expressing the future tense (more common here in the UK than in the US), and so it would be "Ni kantos" in Esperanto.